1. Field of the Invention
The present invention relates to a method for inspecting an outer surface of an object to be inspected and an apparatus therefor. Particularly, the present invention relates to an outer surface-inspecting method and an outer surface-inspecting apparatus suitable for judging whether a defect of a circuit pattern formed on a semiconductor chip is acceptable or not, the defect being caused by a gap of the circuit pattern, a foreign body attached to the circuit pattern, or the like.
2. Description of Related Art
There is used a method using a template as an outer surface-inspecting method for judging whether a circuit pattern is good or not. There is proposed a method including: photographing an outer surface image of one good pattern selected as a template; detecting a pattern shape by carrying out an edge detection process on the photographed image; and automatically dividing an area to be inspected based on the pattern shape into a plurality of sections (e.g., Japan Patent Application Laid-Open No. 2004-85543 (paragraphs 0029-0057 and FIG. 3)). Also, in order to set each section on an area to be inspected according to a detected pattern, there is used a method including: setting layers respectively for inspection conditions based on acceptable levels as to defects; and setting sections respectively having acceptable levels different from each other, which are classified and divided by superposing the layers (e.g., Japan Patent Application Laid-Open No. 2004-132950 (paragraphs 0032-0065 and FIG. 3)).
In such prior art described above, the template and an image obtained from an object to be inspected are compared over each inspection area. When a defect of the object to be inspected is extracted by the comparison, it is judged whether or not the level of the extracted defect exceeds a predetermined acceptable level of a corresponding section. For example, when a spread of the extracted defect is compared to a predetermined acceptable value of a corresponding section where the defect exists and the spread exceeds the acceptable value, the object to be inspected is judged as a defective product.
According to the prior art, set sections can be set automatically by image-processing a photographed image obtained from the object to be inspected. Further, by superposing a plurality of the layers respectively having acceptable levels different from each other, the area to be inspected can be divided into set sections respectively having different acceptable levels, corresponding to a desirable condition. Thus, an operator can easily operate an inspecting apparatus, and a workload of the operator may be reduced.
However, according to the prior art, a centroid method is used for judging whether a defective portion extracted through comparison with the template exceeds a corresponding acceptable level or not. According to the centroid method, as an acceptable level for judging one defective portion consisting of a collective entity of defects, an acceptable level only corresponding to one set section where the centroid of the defective portion is located is chosen.
Thus, when a defective portion spreading out over a plurality of sections respectively having different acceptable levels is found, acceptable levels of sections out of which the centroid is located are not taken into consideration, although one/some parts of the defective portion are located on such sections. That is, when a defective portion spreading out over a plurality of set sections respectively having different acceptable levels exists and the centroid of the defective portion is located on one of the set sections having the lowest acceptable level of all the acceptable levels of the set sections, the lowest acceptable level, i.e., the most strict acceptable level is compared to the defect of the entire defective portion. Similarly, when the centroid of the defective portion is located on another set section having the highest acceptable level, i.e., the lightest acceptable level of all the acceptable levels of the set sections, the lightest acceptable level is compared to the defect of the entire defective portion. Thus, even if there are defective portions respectively having the same area thereof, acceptable levels applied for judgments of the defects of those defective portions may be widely changed according to a slight difference of centroid positions due to a slight difference in shapes/positions between the defective positions and therefore their judgments may result in the opposite.
Even if a defect of a defective portion does not exceed an acceptable level of a set section on which the centroid of the defective portion is located and in the case a part of the defective portion spreads out over a section(s) having an acceptable level more strict than that of the set section where the centroid is positioned, a defect of the part of the defective portion located on the section may exceed the acceptable level of the section on which the centroid is not located. In such case, the acceptable level of the section on which the centroid is not located is not taken into consideration according to the conventional centroid method, thus, it may be overlooked that the defect of the part of the defective portion spreading out over the section on which the centroid of the defective portion is not located exceeds the strict acceptable level of the section out of the centroid. As a result, it may be erroneously judged as a good product.